1. Field of the Invention
The present invention relates to an OLED (organic light emitting device) panel in which a light emitting element such as an OLED formed on a substrate is sealed between the substrate and a cover member, and to a method of driving the OLED panel. The invention also relates to an OLED module obtained by mounting an IC that includes a controller to the OLED panel. In this specification, a light emitting device is used as the generic term for the OLED panel and the OLED module. Also included in the present invention is electronic equipment using the light emitting device.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has made great advancement to promote application of TFTs to active matrix display devices. In particular, TFTs using polysilicon have higher field effect mobility (also called mobility) than conventional TFTs that use amorphous silicon and therefore can operate at high speed. This makes it possible to control pixels, which has conventionally been controlled by a driving circuit external to the substrate, by a driving circuit formed on the same substrate on which the pixels are formed.
With various circuits and elements formed on the same substrate, active matrix display devices can have many advantages including lowering of manufacture cost, reduction in display device size, an increase in yield, and improvement in throughput.
An active matrix light emitting device having an OLED as a self-luminous element (hereinafter simply referred to as light emitting device) is being researched actively. A light emitting device is also called as an organic EL display (OELD) or an organic light emitting diode (OLED).
Being self-luminous, an OLED does not need back light which is necessary in liquid crystal display devices (LCDs) and is therefore easy to make a thinner device. In addition, a self-luminous OLED has high visibility and no limitation in terms of viewing angle. These are the reasons why light emitting devices using OLEDs are attracting attention as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound (organic light emitting material) that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use one or both types of light emission.
In this specification, all the layers that are provided between an anode and a cathode of an OLED together make an organic light emitting layer. Specifically, an organic light emitting layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc.
A basic structure of an OLED is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order, or the like.
A pixel portion of a light emitting device generally has the structure shown in FIG. 21. A pixel portion 1701 is provided with a plurality of gate signal lines 1705, a plurality of source signal lines 1706, and a plurality of power supply lines 1707.
A region that has one of the gate signal lines 1705, one of the source signal lines 1706, and one of the power supply lines 1707 corresponds to a pixel 1702. The pixel 1702 and similarly structured pixels form a matrix in the pixel portion 1701. Each pixel has an OLED 1703. The OLED 1703 has an anode and a cathode. In this specification, the cathode is called an opposite electrode (second electrode) when the anode is used as a pixel electrode (first electrode) whereas the anode is called the opposite electrode when the cathode serves as the pixel electrode.
The opposite electrode in every OLED 1703 receives a given voltage from a power supply 1704 that is external to the OLED panel. The voltage between the opposite electrode and the pixel electrode is called an OLED drive voltage in this specification.
An enlarged view of the pixel 1702 is shown in FIG. 22. The pixel 1702 has the OLED 1703, a first TFT 1708 that functions as a switching element, a second TFT 1709 that controls a current flowing between the pixel electrode and opposite electrode of the OLED 1703, and a capacitor (storage capacitor) 1710.
A gate electrode of the first TFT 1708 is connected to one of the gate signal lines 1705. The first TFT 1708 has a source region and a drain region one of which is connected to one of the source signal lines 1706 for receiving digital signals and the other of which is connected to a gate electrode of the second TFT 1709.
The second TFT 1709 has a source region and a drain region one of which is connected to one of the power supply lines 1707 and the other of which is connected to a pixel electrode of the OLED 1703. Of two electrodes the capacitor 1710 has, one is electrically connected to one of the power supply lines 1707 and the other is electrically connected to the gate electrode of the second TFT 1709.
Next, a method of driving the light emitting device shown in FIGS. 21 and 22 is described. The description here takes as an example gray scale display using n-bit digital signals.
When an image is displayed using n-bit digital signals, one frame period is divided into at least n sub-frame periods. Each sub-frame period consists of a period for inputting digital signals to pixels (writing period) and a period for display by the pixels in accordance with the bits of the digital signals written.
In a writing period, the voltage of the opposite electrode of every OLED 1703 is kept at the same level as the voltage of the power supply lines 1707 by the power supply 1704. The plural gate signal lines 1705 are selected one by one and the first TFT 1708 is turned ON in order when a gate signal line to which its gate electrode is connected is selected. In this specification, a signal line being selected means turning ON every TFT whose gate electrode is connected to the selected signal line.
When digital signals are inputted to the plural source signal lines 1706 separately, the digital signals are inputted to the gate electrode of the second TFT 1709 through the first TFT 1708 that has been turned ON. The voltage of the digital signals is held in the capacitor 1710.
A digital signal has xe2x80x980xe2x80x99 information or xe2x80x981xe2x80x99 information. A xe2x80x980xe2x80x99 signal is a Lo voltage signal and a xe2x80x981xe2x80x99 signal is a Hi voltage signal, or it may be the other way around.
The gate signal lines 1705 are selected one by one until all of them are selected once and digital signals are inputted to every pixel. Inputting a digital signal to a pixel means inputting a digital signal to the gate electrode of the second TFT 1709. A period required to complete inputting digital signals to all pixels in the pixel portion 1701 is called a writing period.
When completing inputting digital signals to all pixels, a writing period is ended to start a display period. As a display period is started, the power supply 1704 changes the voltage of the opposite electrode in each OLED 1703 to generate a voltage between the opposite electrode and the power supply lines 1707.
If a digital signal inputted to a pixel during a writing period has information of xe2x80x980xe2x80x99, the second TFT 1709 is turned OFF and the OLED 1703 does not emit light. On the other hand, if the digital signal has information of xe2x80x981xe2x80x99, the second TFT 1709 is turned ON to give the voltage of the power supply lines 1707 to the pixel electrode of the OLED 1703. Then, the voltage generated between the opposite electrode and the power supply lines 1707 is applied between the pixel electrode and opposite electrode of the OLED 1703, thereby causing the OLED 1703 to emit light.
During a display period, the voltage of the opposite electrode is set to a level that causes application of forward bias voltage to the OLED 1703 as the voltage of the power supply lines 1707 is given to the pixel electrode.
In this way, whether or not an OLED emits light is determined by information of digital signal and all the pixels are used for display at once.
A desired gray scale is obtained by determining for each of the n sub-frame periods whether or not a pixel emits light in the display period.
In a light emitting device that displays an image using digital signals as the one described above, when the light emitting device is increased in size, the number of pixels is increased and a large amount of current flows throughout the pixel portion. This current flows from a power supply that controls the OLED drive voltage and, therefore, a switch the power supply has for controlling the voltage of the opposite electrode has to have high current capacity.
If a luminance of 200 cd/m2 is to be obtained in a light emitting device, the amount of current required is several mA/cm2. For example, when a 40 inch display device is to be manufactured using a 5 mA/cm2 organic light emitting material, the current required to display is about 25 A, which is considerably a large value.
Generally, a given standard current capacity is set for a switch of a power supply, and this upper limit in current capacity is an obstacle for enlargement of the light emitting device.
Moreover, a driving circuit in the above light emitting device has to be operated at higher frequency and one frame period has to be divided into more sub-frame periods as the number of gray scales is increased. The switch frequency characteristic of power supply, on the other hand, tends to decline as the current capacity is enhanced. As a result, the switch frequency characteristic is lowered and the number of gray scales obtainable is reduced as the light emitting device is increased in size.
An object of the present invention is to provide measures for solving the above problems accompanying an increase in size of light emitting device. Specifically, an object of the present invention is to lift current value restriction due to a switch of a power supply that controls the OLED drive voltage and to prevent reduction in number of gray scales by preventing lowering of frequency characteristic of a driving circuit due to the switch of the power supply that controls the OLED drive voltage.
In the present invention, one more TFT is provided between a power supply line and a pixel electrode of an OLED. Specifically, a third TFT is provided for controlling the flow of a drain current of a TFT that is switched by a digital signal into an OLED.
Switching of the third TFT is controlled in each line.
The above structure makes it possible to control the OLED drive voltage while giving an opposite electrode of an OLED a constant voltage. Accordingly, a light emitting device of the present invention does not need a switch of a power supply that controls the opposite electrode voltage and, if it has the switch, the current capacity required of the light emitting device is not high.
Switching of the third TFT can be controlled by a voltage applied to a gate electrode of the third TFT and almost no current flows into the gate electrode of the third TFT.
Therefore, enlargement of the light emitting device is not hindered by upper limit in current capacity of the switch of the power supply for the opposite electrode. Moreover, only a small amount of current flows into the switch of the power supply for the opposite electrode and therefore lowering of frequency characteristic of a driving circuit due to the switch can be avoided to prevent reduction in number of gray scales.
The light emitting device of the present invention may employ a transistor formed from single crystal silicon instead of TFTs. If a TFT is employed, the TFT may be formed of polycrystalline silicon or amorphous silicon. The light emitting device may also employ a transistor formed from an organic semiconductor.